1. Field of the Invention
The present invention relates to a magnetic head of high performance used in various kinds of magnetic recording devices for households, broadcasting stations, etc. More specifically, the present invention relates to a magnetic head in which chip cracks are rarely caused due to improved thermal compatibility between a substrate and sealing glass.
2. Description of the Related Art
In recent years, miniaturized lightweight magnetic heads have been developed along with the increase in recording density of a recording medium such as a VTR. Since it is difficult to manufacture miniaturized lightweight magnetic heads with precision, the manufacturing yield thereof is not high. The reason for such a low yield mainly lies in chip cracks. This is caused by a number of cracks formed in the sealing glass at the time when the magnetic head is formed into chips. Because of this, there has been a demand for a magnetic head with an improved manufacturing yield.
A typical process for manufacturing a magnetic head for a VTR will be described with reference to FIG. 3.
a) A substrate material is cut from a magnetic material such as ferrite ingot;
b) The outside of the cut substrate material is ground in order to obtain a substrate of a predetermined size;
c) Grooves for introducing sealing glass are formed in the substrate;
d) Grooves are filled with sealing glass;
e) Excess glass is removed by grinding, other gaps (grooves) for coiling wire are formed in the resulting substrate, and the gap surface is ground;
f) Gap materials (e.g., SiO.sub.2 or the like) are sputtered on the surface of the two substrates, one substrate for an S-pole is layered on top of the other substrate for an N-pole, and the sealing glass is sealed by heating, whereby a sealed gap is formed;
g) The substrate thus obtained is cut into chips;
h) The side of the chip is ground to a predetermined size;
i) The chip is attached to a base;
j) The surface on which a video tape contact is made polished; and
k) The resulting chip is coiled with wire in order to obtain a magnetic head.
In such a magnetic head, the sealing glass and the substrate have different thermal expansion ratio. Therefore, when the substrate is sealed with the sealing glass at high temperatures, the glass becomes distorted by either tensile stress or compression stress at room temperature. In the magnetic head described above, sealing glass in which compression stress is considered to be applied in a solidified state, i.e., at room temperature is selected. The reason for this is as follows:
Glass is relatively strong against compression stress but relatively weak against tensile stress, and the strength of glass against tensile stress is 1/10 or less that of the strength against compression stress thereof. That is to say, glass is weak against tensile strength, so that glass is likely to be broken due to the generation of tensile stress. From this viewpoint, it is considered that the manufacturing yield can be improved when the sealing glass in which compression stress is applied at room temperature is selected.
FIG. 5 shows the relationship between the thermal expansion ratio and the temperature of four kinds of glass materials, U, V, X, and Y, each having a different thermal expansion profile and that of ferrite used in a magnetic head. Compositions (% by weight) of the glasses U, V, X, and Y are shown in Table 1.
FIG. 6 shows results obtained by sealing ferrite with the glass materials U, V, X, and Y and determining the stress applied at room temperature on the interface between the ferrite and the glass material. In FIG. 6, .alpha..sub.300 denotes an averaged thermal coefficient of expansion in the range of 30.degree. to 300.degree. C. As shown in FIG. 6, in the glass materials X and Y, compression stress is generated; and in the glass materials U and V, tensile stress is generated. In a magnetic head using the glass materials X and Y, chip cracks are rarely caused; and in magnetic heads using the glass materials U and V, chip cracks are more than likely to be caused.
TABLE 1 ______________________________________ X Y U V ______________________________________ Composition by weight (%) B.sub.2 O.sub.3 22 22 15 10 PbO 47 37 35 30 ZnO 9 9 5 5 CdO 7 7 10 20 TeO.sub.2 10 20 30 35 Bi.sub.2 O.sub.3 5 5 La.sub.2 O.sub.3 5 ______________________________________
Thus, the condition for suitable sealing glass is that the sealing glass has a thermal expansion characteristic which causes compression stress within a certain range to be generated at room temperature when used to seal in materials for a substrate (ferrite). This condition is disclosed in Japanese Laid-Open Patent Publication No. 1-138151 by the inventors of the present invention.
If a sealing glass is selected in accordance with this disclosed condition, chip cracks can greatly be decreased for some glass materials; however, chip cracks are still generated depending upon the kind of glass materials. Thus, this disclosed condition is not sufficient for the selection of sealing glass. Even though sealing glass having an averaged thermal expansion ratio in the range of room temperature to a sealing temperature of the sealing glass smaller than that of the substrate is selected, so that compression stress within a certain range is generated at room temperature, chip cracks cannot be reduced.
Moreover, in a metal in gap (MIG) head (described later) which has recently come into use, there arises a problem of the decrease in yield caused by a higher frequency of chip cracks. Conventionally, as a magnetic material for a VTR head, the above-mentioned material such as ferrite has most widely been used. However, ferrite which has recently come into use cannot be applied to high density recording medium such as metal tape, because of the low saturation magnetic flux density thereof. In a high density and high quality recording system for a broadcasting station and for an S-VHS, ferrite cannot be used alone. In these cases, a magnetic substrate obtained by sputtering a sendust alloy, an amorphous alloy, or the like, on a ferrite substrate is used in place of a ferrite substrate. A magnetic substrate which is a ferrite substrate having a super-structured nitride alloy on the surface of the ferrite substrate can also be used. The term "super-structured nitride alloy" is referred to as an multi-layered structure of alloys, the structure having alternate arrangement of amorphous alloy layers and amorphous nitride alloy layers. Usually, the structure is prepared by the sputtering process including alternate sputtering of alloy target in an inert gas atmosphere and in a mixed gas atmosphere containing nitrogen and an inert gas. Generally, the alloy target is M--X wherein M is Co or Fe, and X is at least one selected from the group consisting of Nb, Zn, Bi, and Si. High density and high quality magnetic heads using these magnetic substrates are called MIG heads. The difference in a manufacturing method between the above-mentioned magnetic head and the MIG head is in that the step for forming a magnetic film on the above-mentioned alloy on the ferrite substrate is added between the steps b) and c).
The problem with the MIG head is that the mechanical strength of the sealing glass is further deteriorated compared with a head using ferrite. That is, in the step g) for cutting them into chips, a number of cracks are generated in the sealing glass, resulting in a poor manufacturing yield. In the MIG head in which a magnetic substance obtained by sputtering a sendust alloy, an amorphous alloy, a superstructured nitride alloy, or the like on a ferrite substrate, is used in place of ferrite alone, since the magnetic characteristics of those magnetic materials change due to heat, it is required that the sealing temperature is set at 600.degree. C. or less. For example, when glass having a high PbO content with a low melting point is used, a sealing temperature can be set at 600.degree. C. or less. However, this type of glass with a low melting point has a remarkably deteriorated mechanical strength for sealing, compared with glass having a relatively high melting point for sealing the usual ferrite core, i.e., glass having a sealing temperature in the range of 700.degree. C. to 900.degree. C. For this reason, in the MIG head using glass with a low melting point, a number of chip cracks are generated in the step g) for cutting them into chips. For industrial production of the MIG head, it is required that chip cracks are reduced and the yield is improved.